The present invention relates to electrical connectors and, more particularly, to the electrical interconnection of a multi-row connector to a printed circuit board to provide controlled impedance paths for each circuit interconnection.
Various systems have been used to mate electrical connectors to printed circuit boards or other circuit bearing substrates. In the general case, the connector includes receptacles having pin-like `tail` portions of selected length that are designed to be electrically connected with conductive pads or traces on the printed circuit board. The tail portions can extend linearly from the connectors, that is, along a line co-incident with its receptacle or at an angle relative to its receptacle. Where the connector is mounted on one of the flat sides of the circuit board, the tail portions pass through respective holes in the circuit board and are soldered to conductive pads on the opposite side of the circuit board. Since the connector is mounted on the flat surface of the circuit board, the number of conductive pads on the circuit board can readily match the number of tail portions of the connector. The soldering of the tail portions effects electrical connection and, in many cases, provides an adequate mechanical connection to affix the connector to the circuit board. Where necessary, mechanical fasteners, including threaded fasteners, clamps, brackets, frames, and the like, can be used to assist in effecting the mechanical attachment. The requirements for an edge-mounted connector are somewhat different in that the surface area available at the edge of the circuit board represents a constraint on the total number of circuit board traces and pads that can be provided for interconnection with the connector. The surface-area limitation is exacerbated where the edge-mounted connector is of the multi-row type, that is, parallel rows of pins or receptacles that form a matrix of interconnects. For example, U.S. Pat. No. 4,659,155 to Walkup et al. discloses an edge connector for a circuit board having two rows of receptacles on each side of the circuit board in which equal length tail portions extend from the connector to respective contact pads aligned in a single row on the circuit board. While the Walkup et al. design represents a reasonable solution, this design is limited in the sense that a larger number of rows will increase the number of contact pads in the single row of contacts on the circuit board and increase the difficulty of assembly of the connector to the circuit board and make inspection of the various contacts more difficult.
One of the trends in electronic systems is the development of high speed digital circuits with a requirement for a relatively large number of circuit interconnects between circuit boards. In order to accommodate the requirement for a quantitatively large number of circuit interconnects, high pin count connector systems have been developed which locate the contact devices, either pins or receptacles, on relatively close centers, for example, 0.100 inches (2.54 mm.), in a multi-row matrix so that several hundred or more circuit connects are possible per connector. The tail portions of the connector can be connected to conductive traces on the printed circuit board by a flexible circuit fabricated from laminated Kapton layers with conductive traces extending between connection fields for the `tail` portion of the receptacles of the connector and connection fields for the conductive pads or traces on the circuit board. While flexible circuit assemblies represent a reasonable design approach to the electrical interconnection of the connector and its circuit board, the closely adjacent conductive traces in high pin count applications provide different impedances, including resistive, capacitive, and inductive constituents, for the various circuits. While these impedance differences do not cause performance problems at low-frequencies, the relative impedance differences between circuits can cause problems at relatively high digital speeds, especially where the interconnects are part of a digital bus application in which all pulses must travel in a synchronous or quasi-synchronous manner with minimal relative degradation because of the impedance characteristics of the flexible circuit.